In a shaft system of a permanent-magnet direct-drive outer rotor wind power generator, a bearing is generally provided between a main shaft of a stator and a rotatable shaft of a rotor, and bearing lubricating grease is applied to the bearing.
1. Movement of the Lubrication Grease in the Bearing
With the rotation of a bearing, the movement of the lubrication grease filled in a rolling bearing may be substantially divided into two stages. In general, the amount of lubrication grease filled in the bearing always exceeds the actual required amount of lubrication grease directly participating the rotation of the bearing. At an early operation stage of the rolling bearing, most of the lubrication grease is quickly (less than 1 min) extruded out of a raceway, and the extruded grease accumulates on a cage and in a cavity of the bearing cover and forms a contour outside rollers. In this process, the temperature of the bearing rises rapidly due to the resistance of excess lubrication grease. Most of the excess lubrication grease is extruded out at a beginning stage of the operation, and the extruded lubrication grease near the raceway is likely to be brought into the raceway by the rollers. This lubrication grease is discharged successively little by little as the rollers of the bearing rotate in a cycling manner. At this time, the temperature of the bearing is continuously rising, which is called a running-in stage of the lubrication grease. The running-in stage may last a dozen minutes or even several hours depending on many factors such as the quality and amount of lubrication grease filled in the bearing. When the excess lubrication grease is completely discharged, a small amount of remaining lubrication grease forms thin lubrication grease films on contact surfaces between the rollers, the raceway and the cage by a wedge effect, thus entering a normal operation stage of the bearing. At this time, the temperature of the bearing gradually decreases and reaches a balance state. In other words, a long-term lubrication is mainly functioned by these lubrication films. In addition, during the long-term operation of the bearing, the lubrication grease on the rollers, on the contour near the raceway, and on the cage will be reduced and a part of base oil is formed. When the base oil enters into the raceway, it also assists the lubrication. The capabilities of different greases in the bearing are different in forming contours, and the ideal grease may form a relatively upright contour, and has a running-in stage of short time, and ensure a low temperature and a smooth operation of the bearing in a long-term operation. The capability of forming a channel shape of the lubrication grease is very important, and only the “fluidized” lubrication grease can be ensured to form the channel shape. When the lubrication grease is frozen, or close to the frozen state, the lubrication grease will be accumulated, agglomerated, losing a proper lubrication effect as it should have. The service life of the bearing of the main shaft of direct-drive outer rotor wind power generator may affect the service life of the entire generator. Replacement of the lubrication grease and cleaning the bearing are difficult. In the limit space of partial surrounding area near the bearing, the air temperature of the working environment varies from −45 Celsius degrees to +65 Celsius degrees. Since the lubrication grease may have a low mobility in a low temperature condition, the lubrication grease extruded out of the bearing will hardly return to the raceway.
2. The Analysis of Slip Reasons of the Rolling Bearing
The bearing is coupled to the shall by an interference fit. According to incomplete statistics, coupling failure caused by relative rotation between the shaft and an bearing inner ring accounts for 30% of failure reasons of the shaft type drive parts. In the case that relative rotation presents between the shaft and the bearing inner ring, the journal rubs against the bearing inner ring, causing the size of the journal to be smaller than the size of the inner ring, so that the interference is “lost”. The journal mounting surface is damaged and the inner ring is scrapped.
In the case that the lubrication grease of the rolling bearing is frozen or is no longer “fluidized”, the friction between the inner ring and the rollers cannot overcome the resistance during starting and is apt to cause the slipping of the rollers and their cage as a whole for the significantly increased resistance caused by the state of the lubrication grease between the cage and the rollers. In the case that the lubricant is frozen or is no longer “fluidized”, the main reasons for the slipping of the rollers in a bearing area are as follows: {circle around (1)} the rollers just enters the bearing area, the lubrication grease is frozen, causing the friction coefficient to be smaller; {circle around (2)} the lubrication grease is frozen, causing the rollers to be carried on the cage and further form one piece together with the cage, further resulting in significantly increased resistance; {circle around (3)} a gust of wind comes in a low temperature period of time, and a wind power generator starts quickly, causing too fast variation of the bearing speed, further causing slipping.
When the generator is used in cold winter areas, the bearing lubricating grease between the stator main shaft and the rotatable shaft of the rotor is prone to be frozen after the generator is shut down. In the case that the lubrication grease is frozen or no longer fluidized, force start will cause the slipping of the bearing rollers on the raceway surface when wind comes from the wind field, which makes the rolling friction between the raceway and the rollers turn into sliding friction, and the rollers continue to slip, resulting in deformation of the rollers. In the case that the rollers and the cage are frozen integrally, and the generator is forcibly started when wind comes front the wind field, the rollers and the cage slip integrally, thus the cage may be damaged. Multiple times of continuous such action may cause bearing failure.